Stellar Nucleosynthesis

How Elements from Hydrogen and Helium Are Created

Neon atomic structure, computer illustration.
Neon is created by helium and oxygen. ROGER HARRIS/SCIENCE PHOTO LIBRARY / Getty Images

Stellar nucleosynthesis is the process by which elements are created within stars by combining the protons and neutrons together from the nuclei of lighter elements. All of the atoms in the universe began as hydrogen. Fusion inside stars transforms hydrogen into helium, heat, and radiation. Heavier elements are created in different types of stars as they die or explode.

History of the Theory

The idea that stars fuse together the atoms of light elements was first proposed in the 1920's, by Einstein's strong supporter Arthur Eddington.

However, the real credit for developing it into a coherent theory is given to Fred Hoyle's work in the aftermath of World War II. Hoyle's theory contained some significant differences from the current theory, most notably that he did not believe in the big bang theory but believed instead that hydrogen was continually being created within our universe. (This alternative theory was called a steady state theory and fell out of favor when the cosmic microwave background radiation was detected.)

The Early Stars

The simplest type of atom in the universe is a hydrogen atom, which contains a single proton in the nucleus (possibly with some neutrons hanging out, as well) with electrons circling that nucleus. These protons are now believed to have formed when the incredibly high energy quark-gluon plasma of the very early universe lost enough energy that quarks began bonding together to form protons (and other hadrons, like neutrons).

Hydrogen formed pretty much instantly and even helium (with nuclei containing 2 protons) formed in relatively short order (part of a process referred to as Big Bang nucleosynthesis).

As this hydrogen and helium began to form in the early universe, there were some areas where it was denser than in others.

Gravity took over and eventually these atoms were pulled together into massive clouds gas in the vastness of space. Once these clouds got large enough they were drawn together by gravity with enough force to actually cause the atomic nuclei to fuse together, in a process called nuclear fusion. The result of this fusion process is that the two one-proton atoms have now formed a single two-proton atom. In other words, two hydrogen atoms have begun one single helium atom. The energy released during this process is what causes the sun (or any other star, for that matter) to burn.

It takes nearly 10 million years to burn through the hydrogen and then things heat up and the helium begins fusing together. Stellar nucleosynthesis continues to create heavier and heavier elements, until you end up with iron.

Creating the Heavier Elements

The burning of helium to produce heavier elements then continues for about one million years. Largely, it is fused into carbon via the triple-alpha process in which three helium-4 nuclei (alpha particles) are transformed. The alpha process then combines helium with carbon to produce heavier elements, but only those with an even number of protons. The combinations go in this order:

  • Carbon plus helium produces oxygen.
  • Oxygen plus helium produces neon.
  • Neon plus helium produces magnesium.
  • Magnesium plus helium produces silicon.
  • Silicon plus helium produces sulfur.
  • Sulfur plus helium produces argon.
  • Argon plus helium produces calcium.
  • Calcium plus helium produces titanium.
  • Titanium plus helium produces chromium.
  • Chromium plus helium produces iron.

Other fusion pathways create the elements with odd numbers of protons. Iron has such a tightly-bound nucleus that there isn't further fusion once that point is reached. Without the heat of fusion, the star collapses and explodes in a shockwave.

Physicist Lawrence Krauss notes that it takes 100,000 years for the carbon to burn into oxygen, 10,000 years for the oxygen to burn into silicon, and one day for the silicon to burn into iron and herald the collapse of the star.

Astronomer Carl Sagan in the TV series "Cosmos" describes, "We are made of star-stuff." Krauss notes, "every atom in your body was once inside a star that exploded....The atoms in your left hand probably came from a different star than in your right hand, because 200 million stars have exploded to make up the atoms in your body."